1. Field of the Invention
This invention relates to an image recording apparatus which scans on a photosensitive drum a beam that is controlled, and off, by a data signal corresponding to predetermined image information. The beam records an image, and the inventor can change the on period of the data signal corresponding to the resolution.
2. Description of the Prior Art
A laser printer, as an image recording apparatus, is a recording apparatus in which a laser beam, deflected by a polygon mirror rotating at high speed, forms a latent image on a photosensitive drum by picture elements. The obtained latent image can be developed with a conventional electrophotographic process to form the visible image on copy paper. Since a high speed modulation of the laser beam is possible, a high speed and high quality (high density) printing and graphic recording can be realized.
The laser beam may be modulated by controlling a laser diode or the like, on-off by, a laser modulation pulse which is the data signal corresponding to the image data. When the image data is repeated on and off at every one dot, though the laser modulation pulse is also repeated on and off at every one dot, since the laser beam has some width (diameter) and is moving during the on period of the beam the portion exposed (black) by the laser beam is larger than that not exposed (white), thus the real image has a small white portion. Therefore, fine lines cannot be reproduced finely enough, or if the resolution is decreased for thinning the lines, the fine lines become blurred, and a half tone may be blackened.
Such a trend is more obvious when the resolution is made changeable or when recording, for example, the image of 400 dpi resolution by increasing the frequency of the dot clock while fixing the diameter of the laser beam at 240 dpi resolution, for example, larger than 400 dpi, because the dot distance becomes narrower.
In the past, as disclosed, for example, in Japanese Patent Application Laid-Open No. Sho 59-188675 (1984), it was known to bring the duty ratio of a pulse driving a beam generating means below 50 % to thicken the fine lines in the main scanning direction which is the scanning direction of the laser beam, and leaving the fine lines in the sub-scanning direction orthogonal thereto as it is for preventing the fine lines from becoming unclear. In such case, when the image data is continuously in an on state, there is the possibility that the laser modulation pulse will not become continuous, therefore it has been problematic to obtain the high quality image.